Field
The invention relates to a process for preparing 2-alkyl-4-trifluoromethyl-3-alkylsulfonylbenzoic acids which are useful as intermediates for the preparation of agrochemically active compounds.
Description of Related Art
Agrochemically active compounds for the preparation of which 2-alkyl-4-trifluoromethyl-3-alkylsulfonylbenzoic acids are required are known from a number of documents. Thus, herbicidally active 4-(4-trifluoromethyl-3-thiobenzoyl)pyrazoles are known from WO 2008/125214 A1. Herbicidally active N-(1,3,4-oxadiazol-2-yl)arylcarboxamides, including ones having a substitution pattern in the phenyl ring similar to that in the compounds disclosed in WO 2008/125214 A1, are known from WO 2012/126932 A1.
WO 2008/125214 A1 also discloses a process for preparing the compound 2-methyl-4-trifluoromethyl-3-methylsulfonylbenzoic acid. In this process, 3-fluoro-2-methyl-4-trifluoromethylbenzoic acid is reacted with sodium hydride and sodium thiomethylate to form 2-methyl-3-methylthio-4-trifluoromethylbenzoic acid which is subsequently oxidized to 2-methyl-3-methylsulfonyl-4-trifluoromethylbenzoic acid.
Disadvantages of this process are the use of 3-fluoro-2-methyl-4-trifluoromethylbenzoic acid, which owing to the fluorine substituent on the phenyl ring has to be prepared in a complicated manner, and the introduction of the methyl group by metallation of 3-fluoro-2-methyl-4-trifluoromethylbenzoic acid by means of at least 2 molar equivalents of butyllithium at low temperature, followed by reaction with the toxic methyl iodide. This process is complicated and in addition uneconomical because of the only low yield in the introduction of the methyl group (50.7% of theory).
Processes for preparing substituted benzoic acids by means of transition metal-catalyzed cyanations of chloroaromatics and subsequent hydrolysis of the cyano group to the acid group are likewise known. Palladium and nickel compounds, in particular, are frequently used as catalysts, with nickel catalysts being preferred from an economic point of view because of the significantly lower price. The nickel-catalyzed cyanation of aryl halides by means of sodium cyanide in the presence of Ni(PPh3)3 or [aryl-Ni(PPh3)2Cl] is known, J. Organomet. Chem., 54, 1973, C57. The cyanation of trifluoromethyl-substituted haloaromatics is likewise known. Thus, for example, Bull. Chem. Soc. Jpn. 61 (1988)1985-1990 describes the reaction of meta- or para-chlorobenzotrifluoride by means of potassium cyanide in the presence of NiBr2(PPh3)2 and metallic zinc to form meta- or para-trifluoromethylbenzonitrile. As further ligands, triphenylphosphine (PPh3) and also tri(o-tolyl)phosphine, 1,2-bis(diphenylphosphino)ethane (dppe) and 1,1′-bis(diphenylphosphino)ferrocene (dppf) were used here, with only dppf and PPh3 being found to be effective. CN 102010351 describes a method of preparing 2-fluoro-4-cyanotrifluoromethylbenzene by cyanation of 2,4-dichlorobenzotrifluoride by means of sodium cyanide or potassium cyanide in the presence of NiBr2(PPh3)2. However, the disclosure in the abovementioned documents indicates that the catalytic cyanation of aryl halides often proceeds with unsatisfactory yields.